The present invention relates to an improved rotary vacuum filter and to a rotary vacuum filter discharge flow assembly, and particularly to those types of rotary vacuum filter systems having an axial valve housing located at one end of the filter drum, which maintains a maximum available vacuum and filtrate flow velocity and a substantially reduced level of tubulence and air entrainment.
Rotary vacuum filters have been in use in the pulp and papermaking industry for over 20 years to separate wood pulp from its pulping filtrate liquid. U.S. Pat. No. 3,363,774, for example, covers a conventional pipe machine-type rotary vacuum filter fabricated of metal having an axial valve housing at one end of the filter drum. U.S. Pat. Nos. 4,383,877 and 4,419,165 describe similar rotary vacuum filters which are made of a polymeric material. All of these rotary vacuum filter remain in use today, having mechanical and hydraulic designs little changed in over 50 years.
Conventional vacuum filters typically comprise a rotary drum partially submerged in a tank of pulp slurry. The drum has axially-extending filtrate channels or compartments spaced about its periphery, with such compartments covered by a filter screen. These filtrate compartments communicate through bucket sections, drainage conduits and a frusto-conical contoured annular valve housing in communication with a source of subatmospheric pressure. A stationary valve positioned within a valve seat portion of the valve housing controls the application of subatmospheric pressure to the respective valve chambers of the housing.
As the drum rotates about its axis and subatmospheric pressure is applied, the screen rotates through the pulp slurry and collects a wet mat of fibers from that slurry. When the screen emerges from the tank, filtrate is drawn through the screen and compartments into a filtrate discharge system which removes the liquid from the pulp mat. The filtrate passes through the discharge system including generally hollow bucket sections, drainage conduits and valve chambers, and is then discharged from the vacuum filter through suitable piping. As the drum continues its rotation, the stationary valve member periodically blocks off filtrate compartments from the source of subatmospheric pressure, thereby enabling removal of the pulp mat from the surface of the screen.
The hydraulic design of these conventional rotary vacuum filters often produces a reduced pressure head and a lowering of the available vacuum through the filter system. Excessive turbulent flow of the filtrate is also created and the filtrate flow velocity is decreased thereby lowering the amount of filtrate that can be removed from the system by vacuum filtration. The excessive level of turbulence generally causes an entrainment of excessive amounts of air in the filtrate. Excessive entrained air can have a undesirable effect on the drainage rate of the pulp. For example, the air bubbles produced take on the physical properties of a solid. When these solids are literally filtered out of the filtrate by the fiber mat, an actual blockage of the flow of the filtrate and a lowering of the drainage rate will esult. As the drainage rate is lowered, the washer area required for a particular rate of production goes up, thereby reducing the effective capacity of the vacuum filter system.
To overcome this problem, a higher than optimum inlet pulp consistency is generally required. Entrained air with the same properties as the solids also blocks the flow of the shower water in the sheet and channels it unevenly resulting in less efficient washing. This lowers the liquid displacement ratios of the pulp resulting in higher chemical usage costs. Air entrainment blocks the flow of additional air through the mat during the pulp drying cycle. This results in lower discharge consistencies and a lowering of the driving force of the bleaching agent which in turn lowers the bleaching efficiency and results in higher bleaching chemical costs. Thus, entrainment of air in the filtrate has a major effect on increasing the cost of producing pulp as well as lowering the capacity of the washing equipment.
In some applications, entrained air results in the production of excessive amounts of foam. Conventionally, the entrained air problem has not been overcome by changing the mechanical design of the vacuum filter. Therefore, foam reduction is accomplished chemically by the addition of defoaming agents. However, chemical defoamers are very expensive and result in substantial additional manufacturing costs.
Accordingly, there is a need for a rotary vacuum filter system in which the pressure head and filtrate flow velocity are maintained, and turbulence and air entrainment are significantly reduced, so that a higher quality pulp product can be produced at a higher production rate, in a more efficient manner, and with significantly lower pulping chemical usage.